[exec.then] add missing \exposid

Author: ericniebler

source/exec.tex

@@ -3790,7 +3790,7 @@
     static constexpr auto @\exposid{complete}@ =
       []<class Tag, class... Args>
         (auto, auto& fn, auto& rcvr, Tag, Args&&... args) noexcept -> void {
-          if constexpr (@\libconcept{same_as}@<Tag, @\exposid{decayed-typeof}@<set-cpo>>) {
+          if constexpr (@\libconcept{same_as}@<Tag, @\exposid{decayed-typeof}@<@\exposid{set-cpo}@>>) {
             @\exposid{TRY-SET-VALUE}@(rcvr,
                           invoke(std::move(fn), std::forward<Args>(args)...));
           } else {